1. Field of the Invention
The present invention relates to an electrochemical device such as primary battery, secondary battery (lithium ion secondary battery in particular), electrolytic cell, and capacitor (electrochemical capacitor in particular).
2. Related Background Art
Electrochemical capacitors such as nonaqueous electrolytic secondary batteries like lithium ion secondary batteries, and electrochemical capacitors like electric double layer capacitors have been expected to become power supplies or backup power supplies for small-size electronic devices such as portable devices, and auxiliary power supplies for hybrid cars.
Therefore, the electrochemical devices have been required to have a structure which can easily attain such a smaller size and a lighter weight that they can be accommodated within a limited space where they are to be placed.
Known as an electrochemical device intended to attain such a smaller size and a lighter weight is one having a film-like form as follows.
One having a thin-film structure has been known, in which two composite package films (laminate films) each comprising a synthetic resin layer and a metal layer such as metal foil are overlaid on each other, their edge parts are heat-sealed (thermally fused) so as to produce a lightweight case (sealing bag) employed as an outer container for sealing constituents of an electrochemical device such as a pair of electrodes (anode and cathode) and an electrolyte (see, for example, the nonaqueous electrolytic secondary battery disclosed in Japanese Patent Application Laid-Open No. 2000-294221 and the nonaqueous electrolytic battery disclosed in Japanese Patent Application Laid-Open No. 2000-138040). In this case, the pair of electrodes are connected to respective metal leads each having one end for electric connection and the other end projecting out of the case.
The above-mentioned electrochemical device is mainly constituted by a cathode, an anode, and an electrolyte layer (e.g., a layer made of a liquid electrolyte or solid electrolyte) disposed between the cathode and the anode. Conventionally, the cathode and/or anode has been made by way of the steps of preparing an electrode coating liquid (e.g., in a slurry or paste form) containing an electrode active material, a binder (a synthetic resin or the like), a conductive auxiliary agent, and a dispersant and/or solvent; applying the coating liquid to a surface of a collector member (e.g., a metal foil); and then drying the coating liquid, thereby forming a layer containing the electrode active material (hereinafter referred to as “active material containing layer”) on the surface of the collector member.
In this method (wet method), there is a case where the conductive auxiliary agent is not added to the coating liquid. There is also a case where, instead of the coating liquid, a kneaded product containing an electrode active material, a binder, and a conductive auxiliary agent is prepared without using the dispersant and solvent, and is formed into a sheet by using a hot roll and/or hot press. A conductive polymer may further be added to the coating liquid, so as to form a so-called “polymer electrode”. When the electrolyte layer is solid, there is a case where a method comprising the step of applying a coating liquid to a surface of the electrolyte layer is employed.